The present invention relates to a semiconductor device and more particularly to a device wherein random access memories (RAM) are used for processing images.
One type of RAM used for image processing for displaying characters and images on the screen of a display such as CRT (cathode ray tube) is the serial access memory which is described in, for example "Nikkei Electronics" pp. 219 to 239, issued Feb. 11, 1985 Nikkei MacGraw-Hill. In this RAM, the counter circuit for forming the address signal is included which is made to operate by the control signal and the timing signal which are fed from the exterior terminal so that the selective signal of the word line of the memory array may be formed. Furthermore, data lines of the memory array are connected parallel with the data register via the switching circuit so that data may be serially transferred or received between this data register and the external terminal. In this way, since transfer and reception of data are conducted serially to and from the external terminal, pixel data which is synchronized with the raster scan timing of the CRT can be easily performed. However, though the above-mentioned RAM for processing the image is apparently composed of a RAM, it can perform the action of a shift register having the number of bits which is equal to its memory capacity. Accordingly, each memory is accessed only once each time the number of bits are accessed. As a consequence, when this kind of RAM is used, a problem arises that the image processing action is retarded with regard to preparation and modification of graphics.
Accordingly, it is very desirable to provide a RAM which is more convenient for the image processing. In view of the above fact, the present inventor has suggested constructing a RAM for processing the image (i.e., so-called video RAM) by using a RAM which can to be accessed to in a unit of a plurality of bits (See, for example, "Hitachi IC Memory Disc" issued Sept. 1983, Hitachi Seisakusho K.K.). The above-mentioned 4-bit signals are designed for processing the colored image, each comprising red, blue, green and brightness signals. However, even when such RAM is used, the memory access action and the action of microprocessor over a plurality of cycles are necessary, in which image data is once read out for preparing or modifying graphics and the logical operation is effected between the read image data and the new image data or data on the indicated conditions and then the result is written again into the original memory address.